A friend of mine says all articular adjectives HAVE to be substantival. Is that true? The articular adjective τῷ κρυπτῷ occurs four times in Mat 6:4-6. If it were substantive, it seems that one would have to translate it as something like "the secret place", or "the secret way" but they are not the best fit to the context. I think a better fit would be if this were an article of previous reference referring back initially to the example of "secret" giving in the previous pericope. I can't find any English versions that translate this article, or any commentaries or grammars that address the articles in this passage. I'm stuck and would appreciate any help.

Yes, putting an article in front of anything makes it a substantive, that's a magic power of the Greek article. Sometimes this maps cleanly into English:

οἱ δίκαιοι = the righteous (Matt 13:43)

Sometimes it doesn't:

τὸ ἅγιον = "the holy" = that which is holy (Matthew 7:6)

Grammars often point out that this lets you omit the noun that the adjective describes - "the righteous" can be seen as a shortcut for "the righteous people", "the holy" can be seen as a shortcut for "the holy things".

Think of the public places where hypocrites give to the needy - announcing it with trumpets in the synagogues and on the streets, to be honored by others. That's not where you should give, you should give in "in the hidden (places)".

But that doesn't translate well into English, so translations go with "in private" or "in secret". Incidentally, compare that to the English phrase "in the dark" - you can't say "in dark", the article is needed. I think that's similar to what is going on with the Greek.

This is similar, but with a twist. The hypocrites have their public places where they like to pray in order to be seen by others. We should go into our hidden room instead and close the door (εἴσελθε εἰς τὸ ταμεῖόν σου καὶ κλείσας τὴν θύραν σου), then pray to our father - the one who is "in the hidden" = unseen (πρόσευξαι τῷ πατρί σου τῷ ἐν τῷ κρυπτῷ·).

And πρόσευξαι ῷ πατρί σου τῷ ἐν τῷ κρυπτῷ is is closely related to ὁ πατήρ σου ὁ βλέπων ἐν τῷ κρυπτῷ, which uses the article in a similar way.

Right, articular adjectives in the so-called second attribute position are not substantival. You'd need some special and probably unfalsiable theory of apposition to make them substantival as input into the syntactic construction.

What's going on in Matt 6:4-6 I think is that the article is generic, and the idiomatic translation into English lacks the article. Jonathan's example "in the dark" illustrates an analogous use pretty well in English.

Thanks, this is helpful! I was not aware of Robinson's assertion that articular adjectives can be appositional instead of substantival. Having looked it up, he is referring to cases where the article is repeated, so it does not apply to Mat 6.4 and 6, but it supports my theory that the rule on articular adjectives is not absolute. This has helped me think more freely about this, and oddly, I'm coming around to a substantive intrepretation. Verse 4 is talking about a secret manner of giving and vs 6 is talking aobut a secret place of prayer, so I think I will translate the uses in verse 4 as “in the secret [way]” and those in verse 6 as “in the secret [place]” - both substantive. Thanks again for the help!! I'm still open to more input on this.

I was not aware of Robinson's assertion that articular adjectives can be appositional instead of substantival.

As always the metalanguage is a source of confusion, "appositional instead of substantival" isn't and either/or. A substantive is something that syntactically functions like a noun. Two substantives can be in an apposition. Paraphrase of David Crystal: appositional two constituents at the same level (syntax parsing tree) which have identity of reference.